SQL Masterclass - Top SQL Interview Problems

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Introduction to SQL· 27

Basics of SQL

What is SQL?
What is a query?
Query Definition
Why SQL Matters
Divider
Syntax in SQL
SQL Syntax Introduction
Advanced Note
SQL Introduction

Sample Database - College Management System

Table 4: enrollments
Scenario: Your College Database
Professors Table
Table 3: courses
Courses Table Description
Database Scenario
Divider
4 Simple Tables
Table 1: students
Students Table Description
Students Table
Table 2: professors
Professors Table Description
Courses Table
Enrollments Table Description
Enrollments Table
Learning Quote

Basic SQL Queries· 87

Introduction to SQL Commands

Types of SQL Operations
SQL Types Introduction
SQL Categories
Learning Note

CREATE - Creating Tables

Constraint Note
CREATE Command
CREATE Introduction
CREATE Explanation
CREATE Requirements
CREATE Requirements List
Syntax Introduction
CREATE TABLE Syntax
Example Heading
Create Students Table
Divider
Adding Constraints
Constraints Explanation
CHECK Constraint
CHECK Constraint Example
Common Constraints
CREATE Table Quiz

INSERT - Adding Data

INSERT Command
INSERT Introduction
INSERT Requirements
INSERT Keywords
Basic Syntax
INSERT Syntax
Column Names Optional
Example
INSERT Students Example
INSERT Tips
INSERT Data Quiz

SELECT - Reading Data

SELECT Specific Columns
SELECT Command
Reading Data Introduction
SELECT Components
Syntax
SELECT Syntax
Example Introduction
SELECT All Explanation
SELECT All Example
Best Practice
SELECT Quiz

WHERE - Filtering Data

WHERE Clause
WHERE Introduction
What WHERE Does
WHERE Explanation
Syntax
WHERE Syntax
Common Operators
WHERE Operators
Divider
Examples
Example 1 Introduction
CSE Department Example
Example 2 Introduction
AND Operator Example
Example 3 Introduction
IN Operator Example
Example 4 Introduction
IS NULL Example
Example 5 Introduction
BETWEEN Operator Example
Example 6 Introduction
Complex Conditions Example
Parentheses Tip
WHERE Quiz 1
WHERE Quiz 2

DISTINCT - Removing Duplicates

Without DISTINCT
Result Without DISTINCT
With DISTINCT
Result With DISTINCT
Performance Tip
DISTINCT Quiz 1
Syntax
Comparison Introduction
DISTINCT
DISTINCT Introduction
DISTINCT Syntax
Example
DISTINCT Department
Divider
DISTINCT with Multiple Columns
Multiple Columns Explanation
DISTINCT Multiple Columns
Important Note
With vs Without DISTINCT

Grouping & Sorting· 75

ORDER BY - Sorting Results

ORDER BY
ORDER BY Introduction
Syntax
ORDER BY Syntax
Examples
Sort by CGPA
ORDER BY CGPA Example
Multiple Column Sort
Multiple Column Sort Example
ORDER BY Quiz

GROUP BY - Grouping Data

Divider
Core syntax
GROUP BY Syntax
Divider
Starter examples (our college DB)
Example 1 - Unique departments
Unique Departments
Example 2 - Finer buckets
Department + Year Grouping
Divider
DISTINCT vs GROUP BY (common doubt)
GROUP BY
Why GROUP BY?
Examples
Example Questions
Divider
What does "grouping rows" mean?
Grouping Explanation
Mini students table
Divider
Step-by-step mental model
Step 1 - Find the categories
Step 1 Details
Bucket Breakdown
NULL Values in GROUP BY
When to use what
DISTINCT Example
GROUP BY for Grouping
Divider
Rules you must remember (or you'll get errors)
Column Selection Rule
Correct Example
Incorrect Example
GROUP BY Quiz 1
GROUP BY Quiz 2

Aggregate Functions - Summarizing Data

Using with GROUP BY
Pro Tips
Aggregate Functions
What are Aggregate Functions?
Basic Example
Common Aggregate Functions
Function Reference
Employee Table Example
Department-wise Statistics
Aggregate Functions Quiz
GROUP BY Quiz

HAVING Clause - Filtering Groups

What Exactly Gets "Grouped"?
Combined WHERE and HAVING
HAVING Clause
What is the HAVING Clause?
Simple way to remember
Why We Need HAVING
WHERE Clause Limitation
❌ Wrong Way - WHERE with Aggregate
✅ Correct Way - HAVING
Grouping Explanation
Step-by-Step Execution
Execution Steps
Tiny Example
Key Takeaways
Key Takeaways List
Multiple Grouping Example
Combining WHERE and HAVING
HAVING Clause Quiz
WHERE vs HAVING Quiz

SQL Practice Queries· 18

Query 3 Description
Practice Queries
Introduction
Database Reference
Practice Query 1
Query 1 Description
Query 1 Solution
Practice Query 2
Query 2 Description
Query 2 Solution
Practice Query 3
Query 3 Solution
Practice Query 4
Query 4 Description
Query 4 Solution
Practice Tips
Practice Query Challenge
Aggregate Functions Quiz

SQL Constraints· 56

Introduction to Constraints

SET NULL Action
SQL Constraints
Types of Constraints
Constraint Types Overview
Introduction
Database Reference
Common CHECK Patterns
SET NULL Explanation
SET NULL Example
RESTRICT Action
RESTRICT Explanation
RESTRICT Example
CHECK Patterns Examples
Quiz: NOT NULL vs UNIQUE
Quiz: PRIMARY KEY
Quiz: FOREIGN KEY CASCADE
Quiz: CHECK Constraint
Quiz: DEFAULT Constraint
Quiz: Referential Integrity

NOT NULL and UNIQUE Constraints

NOT NULL Constraint
NOT NULL Explanation
UNIQUE Constraint
UNIQUE Notes
UNIQUE Explanation
Quiz: NOT NULL vs UNIQUE
NOT NULL Example
UNIQUE Example

PRIMARY KEY Constraints

PRIMARY KEY Explanation
PRIMARY KEY Example
PRIMARY KEY Rules
Quiz: PRIMARY KEY
PRIMARY KEY Constraint

FOREIGN KEY Constraints

FOREIGN KEY Constraint
FOREIGN KEY Definition
Quiz: FOREIGN KEY CASCADE
FOREIGN KEY Introduction
FOREIGN KEY Basic Example
Referential Actions
CASCADE Action
CASCADE Explanation
CASCADE Example

CHECK and DEFAULT Constraints

Quiz: CHECK and DEFAULT
CHECK Constraint
CHECK Explanation
CHECK Example
DEFAULT Constraint
DEFAULT Explanation
DEFAULT Example

Advanced Constraints & Best Practices

Quiz: Referential Integrity
Altering Constraints
Best Practices
Adding and Dropping Constraints
Altering Constraints Examples
Constraint Best Practices
Quick Recap
Recap

SQL JOINS· 88

Introduction to JOINS

SQL JOINS - Complete Guide
Types of JOINS
Why We Need JOINs
Example Scenario
JOIN Types Visual Guide
JOIN Types Comparison
What is a JOIN?

INNER JOIN

INNER JOIN
INNER JOIN Syntax
INNER JOIN Syntax Example
INNER JOIN Example
INNER JOIN Example Query
Important Note
Query Result
What is INNER JOIN?
Simple Explanation
How JOINs Work
Example Question
INNER JOIN Quiz

LEFT JOIN (LEFT OUTER JOIN)

What is LEFT JOIN?
How LEFT JOIN Works
Syntax
LEFT JOIN Syntax
Example 1: All Students, Including Those Not Enrolled
Example 1 Question
Example 1 Query
Example 1 Result
Result Explanation
Key Point
Example 2: Courses with Enrollment Count
Example 2 Question
Example 2 Query
Example 2 Result
LEFT JOIN Quiz

RIGHT JOIN (RIGHT OUTER JOIN)

Syntax
RIGHT JOIN Syntax
Example: All Courses, Even Without Enrollments
RIGHT JOIN Example Query
RIGHT JOIN Result
Pro Tip
Same Query Using LEFT JOIN
LEFT JOIN Alternative
RIGHT JOIN Quiz
RIGHT JOIN (RIGHT OUTER JOIN)
What is RIGHT JOIN?
RIGHT JOIN vs LEFT JOIN

FULL OUTER JOIN

FULL OUTER JOIN Result
Result Explanation
FULL OUTER JOIN Quiz
What is FULL OUTER JOIN?
How FULL OUTER JOIN Works
Simple Explanation
Syntax
FULL OUTER JOIN Syntax
Example: All Students + All Courses
Example Goal
FULL OUTER JOIN Example Query
FULL OUTER JOIN

CROSS JOIN

What is CROSS JOIN?
Simple Explanation
Cartesian Join
Syntax
CROSS JOIN Syntax
CROSS JOIN
Example: Every Professor × Every Course
Example Question
CROSS JOIN Example Query
CROSS JOIN Result
Result Explanation
Where is CROSS JOIN used?
Use Cases Introduction
CROSS JOIN Use Cases
Important Warning
CROSS JOIN Quiz

SELF JOIN

How SELF JOIN Works
Example 1: Professor Mentors
Example 1 Scenario
Example 2: Students in Same Department
SELF JOIN
What is SELF JOIN?
Simple Explanation
SELF JOIN Example 1 Query
Example 1 Result
Example 2 Question
SELF JOIN Example 2 Query
Example 2 Result
Key Points
SELF JOIN Quiz

SQL Set Operations· 33

Introduction to Set Operations

What Are Set Operations?
Mathematical Set Operations

UNION - Combine Unique Results

UNION - Combine All Results
How UNION Works
Example 1: All Student Names and Professor Names
Example 2: All Departments (Students + Professors)
Result
Syntax
Solution
Solution

UNION ALL - Combine All Results

Key Difference
UNION ALL - Combine All Results (Keep Duplicates)
Result
Example: All Departments (Including Duplicates)

INTERSECT - Common Elements

INTERSECT - Common Elements Only
Example: Students Who Are Also Teaching Assistants
Result
Example: Common Departments
Result
Solution
Solution

EXCEPT - Elements from First Set Only

EXCEPT (MINUS in Oracle) - Elements from First Set Only
Example: Students Not Teaching
Result
Example: Departments with Only Students (No Professors)
Result
Solution
Solution

Advanced Set Operations

Advanced Examples - Complex Analysis
Quick Comparison Table
Important Rules for Set Operations
Practice Exercises
Complex Analysis Query

SQL Subqueries· 29

Introduction to Subqueries

What's a Subquery?
Definition
General Syntax
How Subqueries Work
Common Clauses Where Subqueries Are Used
Real-World Example
Example Query
Explanation

Single-Row Subqueries

Example: Find students who joined after the first admission date
Single-Row Subquery

Multi-Row Subqueries

Multi-Row Subquery
Example: Find students who live in the same city as professors
Explanation

Correlated Subqueries

Correlated Subquery
Example: Find students whose CGPA is above the average CGPA of their department
Explanation

Subqueries in Different Clauses

Subquery in FROM Clause (Inline View)
Example: Find average CGPA per department and show only those above 8
Explanation
Subquery in SELECT Clause (Scalar Subquery)
Example: Show each student with total student count next to their name
Subquery with EXISTS
Example: Find students who have at least one enrollment
Explanation

Subqueries vs JOINs - Best Practices

Types of Subqueries
Subquery vs JOIN Comparison
Use Subquery when:
Use JOIN when:
Pro Tip